Update particles_run call to allow accurate particle advection using u, v or uh, vh

The drifters package is able to run in 2 different modes: one where the particles are advected using u and v, and one where they are advected using uh and vh. When u and v are used (i.e. the drifters are advected using the resolved velocities only, with no sub-grd component), the particles package needs the layer thickness that is consistent with these velocities, so particles_run needs to be called before any subgrid scale parameterizations are applied. This was not implemented correctly in my last PR, and is corrected here.

The particles package also needs the correct timestep for particle advection. This is added here.

Author: cspencerjones

config_src/external/drifters/MOM_particles.F90

@@ -28,7 +28,7 @@ subroutine particles_init(parts, Grid, Time, dt, u, v, h)
 end subroutine particles_init
 
 !> The main driver the steps updates particles
-subroutine particles_run(parts, time, uo, vo, ho, tv, use_uh, stagger)
+subroutine particles_run(parts, time, uo, vo, ho, tv, dt_adv, use_uh)
   ! Arguments
   type(particles), pointer :: parts !< Container for all types and memory
   type(time_type), intent(in) :: time !< Model time
@@ -40,8 +40,8 @@ subroutine particles_run(parts, time, uo, vo, ho, tv, use_uh, stagger)
                                            !! that are used to advect tracers [H L2 ~> m3 or kg]
   real, dimension(:,:,:), intent(in) :: ho !< Ocean layer thickness [H ~> m or kg m-2]
   type(thermo_var_ptrs),  intent(in) :: tv !< structure containing pointers to available thermodynamic fields
+  real, intent(in) :: dt_adv !< timestep for advecting particles [s]
   logical :: use_uh !< Flag for whether u and v are weighted by thickness
-  integer, optional, intent(in) :: stagger !< Flag for whether velocities are staggered
 
 end subroutine particles_run
 

src/core/MOM.F90

@@ -1279,6 +1279,18 @@ subroutine step_MOM_dynamics(forces, p_surf_begin, p_surf_end, dt, dt_thermo, &
 
   endif ! -------------------------------------------------- end SPLIT
 
+  if (CS%use_particles .and. CS%do_dynamics .and. (.not. CS%use_uh_particles)) then
+    if (CS%thickness_diffuse_first) call MOM_error(WARNING,"particles_run: "//&
+      "Thickness_diffuse_first is true and use_uh_particles is false. "//&
+      "This is usually a bad combination.")
+    !Run particles using unweighted velocity
+    call particles_run(CS%particles, Time_local, CS%u, CS%v, CS%h, &
+                       CS%tv, dt, CS%use_uh_particles)
+    call particles_to_z_space(CS%particles, h) 
+  endif
+
+
+
   ! Update the model's current to reflect wind-wave growth
   if (Waves%Stokes_DDT .and. (.not.Waves%Passive_Stokes_DDT)) then
     do J=jsq,jeq ; do i=is,ie
@@ -1364,23 +1376,21 @@ subroutine step_MOM_dynamics(forces, p_surf_begin, p_surf_end, dt, dt_thermo, &
   endif
   call disable_averaging(CS%diag)
 
+  ! Advance the dynamics time by dt.
+  CS%t_dyn_rel_adv = CS%t_dyn_rel_adv + dt
+
   if (CS%use_particles .and. CS%do_dynamics .and. CS%use_uh_particles) then
     !Run particles using thickness-weighted velocity
     call particles_run(CS%particles, Time_local, CS%uhtr, CS%vhtr, CS%h, &
-        CS%tv, CS%use_uh_particles)
-  elseif (CS%use_particles .and. CS%do_dynamics) then
-    !Run particles using unweighted velocity
-    call particles_run(CS%particles, Time_local, CS%u, CS%v, CS%h, &
-                       CS%tv, CS%use_uh_particles)
+        CS%tv, CS%t_dyn_rel_adv, CS%use_uh_particles)
   endif
 
-
-  ! Advance the dynamics time by dt.
-  CS%t_dyn_rel_adv = CS%t_dyn_rel_adv + dt
   CS%n_dyn_steps_in_adv = CS%n_dyn_steps_in_adv + 1
   if (CS%alternate_first_direction) then
     call set_first_direction(G, MODULO(G%first_direction+1,2))
     CS%first_dir_restart = real(G%first_direction)
+  elseif (CS%use_particles .and. CS%do_dynamics .and. (.not.CS%use_uh_particles)) then
+    call particles_to_k_space(CS%particles, h)
   endif
   CS%t_dyn_rel_thermo = CS%t_dyn_rel_thermo + dt
   if (abs(CS%t_dyn_rel_thermo) < 1e-6*dt) CS%t_dyn_rel_thermo = 0.0